Mechanism generally known as bowden mechanism



R. TONDEUR Nov. 21, 1939.

MECHANISM GENERALLY KNOWN AS BOWDEN MECHANISM s Sheets-Shet 1 Filed Nov. 8, 1938 R. TONDEUR 2. 180,442

Nov. 21, 1939.

' MECHANISM GENERALLY KNOWN AS BOWDEN MECHANISM 5 Sheets-Sheet 2 Fil ed Nov. 8, 1938 I nventor NOV. 21, 1939. TQNDEUfQ 2,180,442

MECHANISM GENERALLY KNQWN AS BOWDEN MECHANISM Filed Nov. 8, 1938- 3 Sheets-Sheet 3 Inventor Kfi azwflw Attorney Patented Nov. '21, 1939 UNITED STATES MECHANISM GENERALLY IBOWDEN MEOHANIS PATE NT. OFFICE,

mews A8 M Rene Tondenr, London, England Application November a, 1938, Serial Ne. 239,416 In Great Britain A t-1111,1938

Claims. (01. 74-501 10 during the transmission of such power, and the mechanism being adapted to transmit such power even when the tubing is curved.

' As usually'constructed the inner member of such Bowden mechanism consists ofa flexible 5 cable formed by surrounding a solid central core by a plurality of concentric layers of helicallylaid coarse pitched single wires or groups of wires, -whilst the outer member usually comprises a flexible tubing or cylindric casing either formed 'of a closely coiled single wire like a helical extension spring, ,or composed of two or more closely interwoundrelatively displaceable coils of such section that when such tubing is,

flexed the coils mutually interactto cause the tubing to expand and contract along the tension and compression sides of the bend re' spectiv'ely, so that the effective axial length is "maintained. substantially constant, and that when compressive axial forces are applied to the tubing it remains sufliciently incompressible for tageous forms of such last mentioned tubing for Bowden mechanism see the specification of my prior Letters Patent Number 2,092,898. a

It'is known that the co-eflicient of friction between the inner and outermembers of Bowden mechanism tends to rise in use despite lubrication, owing; it is believed; to the expulsion of the lubricant from between the relatively movable "surfaces as those are locally pressed strongly towards each other round bends so that the surfaces eventually approach each other very closely and by adhesionbetween them increase their frictional resistance to relative motion. In other words the pressure per square inch between the surfaces in contact is so high that'the oil film breaks with consequent damage to the contacting surfaces.

The greater the pressure per unit area over the contacting area when the members are pressed togetherthe greater is the tendency for these -to seize and to abrade and wear through, and up to-now even the use of carefully selected lubricants has failed to overcome permanently this difficulty owing to the limited radical elastic deformation and flexibility obtainable in the inher member hitherto employed.

Moreover in wire cables having a plurality of. concentric layers of helically-laid wires, there is relative movement of the respective shells of the'efiective transmission of power. For advan-' the (table when this is flexed. Consequently there are considerable frictional losses owing to the rubbing of the wires of one shell against the wires of a neighboring shell, whilst the solid core also offers resistance to bending.

t It' will be seen therefore that the working of Bowden mechanism i'snpen to be improved, (1) by increasing the area of contact and hence reducing the pressure per unit area exerted between the inner and outer members where these are. pressed togetherespecially round bends and so reducing 'the'reciprocal abrasive action of the relatively mfovable inner and outer members on one another and' (2) by reducing the resistance to flexure of the inner member by reducing the internal frictional losses there- 'of by avoiding a plurality of layers and undue stifinessin the core.

If it were possible to employ an inner member of the same diameter as the inside diameter of the outer member the ideal of a very considrerable area of surface contact, probably of 180, would be attained. This is not attainable in practice but the nearer the diameter of the inner member approaches the inside diameter of the Outer member with due regard tothe clearance necessary for receiving lubricant, the nearer can this ideal be approached provided adequate radial elastic deformation of the inner member is permitted to enable this to make substantial surface -contact with the outer member under the workeconomic extent, but the increase'in diameter of the inner member would be attended by a very unwanted increase in its resistance to flGXUIB.

The present invention aims in particular, to overcomexthese disadvantages by a construction of inner member which shall be readily flexible and shall be characterised by reduced frictional losses not only between its own wires but also between itself and the outer compression resis ing casing of the Bowden mechanism:

To these ends, the present invention consists in having the inner tension member composed of a single layer or shell of helically-laid coarse pitched wires surrounding a hollow core in the form of aflne pitched open coil so as to form .core fitting or approximately fitting the interior of the shell so as to afford adequate support to a hollow structure which is readily transversely the latter. This transverse deformation permits of an increased area of contact with the outer member when the member is curved.

In this way when the inner and outer members are pressed together round bends the area of surface contact between the inner and outer mem- 'Itis essential that the hers is increased ami moreover where the mech-' anism is curved there is little or no relative longitudinal movement between the cor and the encasing shell of the inner member. It is not essential to secure the ends of the core and surrounding shell together unless the end attachments are joined to the shell through theintermediary of the core as the tension forces are takenby the outer shell. F 9

core be of open coiled wire. The spacing between the adjacent convolutions may be small but it should be such that when the inner mem er has been bent to' the predetermined maximum extent the convolutions do not strongly press against each other on the inner, i. e., the compression. side of the bend. The axial length of the core will thus remain the same whether the core be straight or curved. The minimum mean radius to which the mechanism can be safely set depends upon the construction and flexibility of the outer member which is the part of the mechanism which is most liable to break as a result of repeated ilexur'es if the radius of curvature be too small. This minimum I working radius :r in the case of the well known Bowdenex mechanism (U. S. Patent No. 2,092,-

v898) has been established at approximately x: 16D, where D is the external diameter of such filter Bowden member. In the older type of wden mechanism in which the outer member consis f a single closely wound coil the mini-' -mum orking radius is generally somewhat greater.

The spacing. [1] between the adjacent convolutions of the core wire when the core is.

straight can be determined-by the formula where a represents the width of thewire, i. e., the

dimension parallelto the axis of the inner memher, or when the wire is of circular cross section, its diameter, and (1 equals the external diameter of the core.

In order that the present invention may be the more readily understood, reference is hereinafter made to the constructional form of flexible inner member and diagrammatic views of forms of Bowden mechanism under the present invention illustratedby way of examplein the accompany:-

ing drawings, in which:

Fig. 1 is a side elevation of a portion of a -metallic inner member embodying the present invention, part of the shell being broken away to reveal the open coiled wire forming the supporting As shown in the accompanying drawings, a substantially inextensible flexible inner member which is found to work well for thefpurpose of the present invention is composed of a single layer or shell 1 of coarse pitched plow steel wires of round section surrounding a hollow core 2 of open coiled fine pitched wire 6f plow steel and rectangular section. a

- When I speak of fine pitch in the specification and claims I mean that the wire of the core is so wound that the angle cnade by the wire with the axis does not go below sixty degrees, and preferably dbes not go below between seventy and eighty degrees and when I speak 'of coarse pitch I mean that the wires are .so laid or twisted that the angle made by the wire with the axis of the core does not exceed 30 and preferably does not exceed 25.

The sectional shape of the fine pitched wire and the coarse pitched wires may vary but I have obtained good results with rectangular section wire, the longer side being preferably parallel to the axis, for the core and round section wires for the shell or shells.

In the diagrammatic examples shown by Figs. 3, 4 and 5, of Bowden mechanism adapted to work in the manner of Bowden mechanism of the kind to which, the present inventionrefers, 3 is the inner tension member and 4 is the outer compression resisting member. In Figs. 3, 5 and 6 are stops by which the ends of the outer member are restrained from axial movement in either direction, 1 an actuating lever to which the inner tension member is attached at one end and 8 represents the object to be moved to which the inner tension member is attached'at its other end. In Fig. 3, the mechanism operates by a pulling motion of the inner tension member 3 relatively to the stationary outer member 4.

Fig. 4 differs from Fig. ,3 in that the ends of the inner tension member 3 areanchored at 9, l0,

- and the object 8 to be moved is displaced by a pushing motion of the outermember 4 relatively to the stationary inner tension member 3 by actuating the lever l.

, In Fig. 5 one end of the inner tension member 3 is anchored at l I while the other end is attached to the lever 1. One end of'the outer member 4 is restrained by the stop 6 while its otherend is free to displace the object 8 as the outer member tends to straighten itself as tension is applied to the inner member by actuating th'e leverl.

. As a comparative example of the usual Bowden mechanism of the kind referred to, an outer member of .223' of aninch inside' 'diameter'and an inner member of .192 of an inch outside diameter composed of thirty-six plow steel wires of .027 of an inch diameter and to tons tensile strength'have been replaced with good results by an outer member of .270 of an inch inside diameter and an inner member of one quarter of an inch outside diameter composed of seventeen round section plow steel wires of .035 inch diamsizes of Bowden i'nnermembers .madeaccording to the usual method and of corresponding memmechanism and the present improved Bowden bers according tothis invention which I have found suitable to replace them in practice, the correspondence being indicated by the numbers.

Inall cases the innermembers were subjected width of one and one quarter inches.

to a transverse loading oi 160 kiiogrammes, on a 3 section wire for the core when making inner Bowden members of the smallerjdiameters and for this reason the sizes 01! rectangular section wires are not stated in the in thetables.

Diameter Trans No External 0! coarse M verse diameter pitch Area oiindividual wiresxNo. of wires defors mation Inch Inch 'Inch 125 015 M1707X36 M03612 J. 0176 159 021 M3404X36 0124704 A 0192 .250 .035 .M9621X36m0340350 .0248

' .311 .047 .001735 xaa=.ooz4eo .0212

, members tabled above Diameter External Trans- No External oicoarse WELL; diameter Section of verse diameter pitched AreaoiindividualwiresXNo.oiwires oihollow core deforwires core mation Inch Inch Inch Inch Inch 156 .021 .'0003404"Xl8-.(X)02352 114 .0225 dia.) .020 .188 .027 .0005725"Xl8-.0l03058 .134 .0225 dia.) .023 .250 .035 .0009021"X18-.0173l78 .180 028 dis.) .025 .312 .047 .oo1735" X18-.031230 .218 .028 dia.) .020 .312 .047 .001735" Xl8=.031230 .218 .024X.035 .0270 .395 .000 mass" ,X18=. 05184 .275 .032 (618.) .038 .395 .060 111288" X18=.05l84 .275 .024X.035 .0392

,the above mentioned load is applied, and the re- .ductions' so shown give an approximateindication of the reductions which will take place in the diameter of the innertmember whichis compressed when the Bowden mechanism is bent under working tension.

The number ofturns tothe inch of the core wire of each of the above inner members accord-.

, ing to the present invention as in the order above given are as follows? -I'he'respective outside andinside diameters of the outer members for-the concentric-lay inner membersand of the'outer members for the, cor

responding new inner membersare} External diameter 1 External Outside oi substi- Outside diameter diameter inside tuted diameter Inside of mm of cones, diameter, inner 1 of cordiamecentricponding thereof members respondr lay inner outer according ingouter thereof member member tothe member 7 present invention g It is generally more economical touse circularwhere an inner member according to this invention is to have the same elongation I characteristic as an inner member constructed according to former practice, the total cross-sectional area of. the coarse pitched wires of the shell according to this invention should be substantially equal to the total cross-sectional areaof the concentric wires in the older form which thenew inner member is intended to replace, it will be seen from the examples given above that it is generally practicable to work with an inner member according, to this invention in which the total crosssectional area of theJcoarse pitched ires is -less than that given by such rule, and this difference may be attributable, or partly attributable, to the greater pitch of these wires in the new form.-

Greater transverse deformation and larger surface contact is generally attained iithe shell is composed of. a greater number oi wires of smaller cross sectional area. .The use of '17 or 18 wires be foundto be generally advantag'eous on the score of economy, but a smaller number can be used, e. g., asfew as 12, or on the contrary where a light and strong structure is more important than economy in the cost or pro- "duction, e. 5., where large tubings are involved which require to resist great pressures, considerably more wires of smaller cross section can bemust not press strongly against each other when the mechanism has been curved to the minimum working radius, it is advantageous that such convolutions should then be contacting or nearly contacting; M v a The difllculty of ensuring that a,user oi Bowden mechanism will use the proper-lubricant necesfirst three examples Although 'it would seem that theoretically I Usual concentric .lau Bow'den inner meinber sary to obtain uniform results over an extended period'can be overcome by the present invention, because I can so-control the extent or the surface contact by adjusting the radial elasticity or transverse deformation ,of the inner member,

that any good lubricant will serve, whether graphitedor not, as the pressure; per unit areais reduced to a point where it is'most unlikely to break the oil film.

By the present invention also I am able to reduce the energy expended in operating the mech-' anism itself when transmitting power.

An inner member made according to the present invention preserves its axial length substantially constant when the member iscurved and works well with Bowden outer tubing formed of closely coiled single wire or better still with Bowden -outer tubing composed of two or more. interwound relatively displaceable calls as described in myiprior Letters Patent No. 2,092,898.

. In general compared with the sizes of the inner and outer members of Bowdenmecha'nism which have hitherto usually been employed for a given load, I increase the diameter of the inner member without increasing the diameter of the outer member in the same proportion.-

' Asan example compared with Bowden mecha-' nism now being made and sold under the registered Trade-Mark Bowdenex by Bowden Engineers Limited, according to Catalogue 193'7\ Data sheet No. 1, I have obtained excellentresults by replacing outer member (conduit) No; T. 6

having an .overall diameter of .441 of an inch and an inside diameter of .223 of an inch and an inner member (cable) No; 36/6 having an overall diammounted so that when the power is transmitted said displacement the outer member is subjected to axial compressive forces and the inner member to axial extension forces, said inner tension member being composed of a single shell of helically laid coarse-pitched wires and a hollow core in the form of a fine pitched open coil substantially fitting the interior of said shell to aflord'adequate support to the latter so that the said inner I wire of which is wound tomake an angle isunder working tension.

2. Bowden me anism as in claim" 1,.having the core wire of rectangular section with the longer side-of th rectangle parallel to the axis.

3. Bowdehmechanisin comprising an outer member and an inner member and adapted to transmit power by relative longitudinal displacement of said members, said members being mountedso that when the power is transmitted by said displacement the outer member is subjected to axial compressive-forces and the inner member to axial extension forces, said innertension. member being composed of a single shell of helically laid coarse-pitchedwires and a hollow core in the form of a fine pitched open coiledthe th the axis of the coil not less than 70, said coil substantially fitting the interior of said shell to afford adequate support to the latter so that the said inner member forms a hollow sructure which is flexible and readily transversely deformable when member and an inner member and adapted to transmit power by relative lon tudinal displacement of said members, sai members being mounted so that when the power is transmitted by said displacement the outer member is sub-- iected to anal compressive forces and the inner member to axial extension forces, said inner tension member being composed of a single shell of wires laid helically in a coarse pitch so as to make an angle which is not greater than 25 with the axis of said shell and a hollow core in .thevform of a fine pitched open coil substantially fitting the interior of the shell to afford adequate support to the latter so that the said inner member forms a hollow structure which is fiexible and readily transversely deformable when the mechanism is curved and sucii inner member is under working tension.

5. Bowden mechanism according to claim 4, in which the wire of said open coil is wound so as to 'make anangle with theaxis of the coil not less than 70.

RENE TONDEUR. 

